This invention relates to a system for converting the optical image of a laboratory slide to electrical signals and more particularly to a pulse discriminator circuit employed in an acquisition system for bringing a specimen of interest on the slide into position for analysis.
In the analysis of blood samples, the blood is smeared on a laboratory slide and the smear is stained. By counting the leukocytes on the stained smear, laboratory technicians perform what is referred to as a white blood cell differential. Automation of this differential has significant economic impact because the differential is performed so frequently at every hospital. A thesis by J. W. Bacus, "An Automated Classification of the Peripheral Blood Leukocytes by Means of Digital Image Processing", University of Illinois, Chicago, 1971, describes one automated system.
In a system disclosed in U.S. Pat. No. 3,883,852, D. A. Cotter, a scanning unit (in this case a T.V. camera) linearly sweeps a vidicon target subjected to intense illumination which passes through the smeared slide.
In order to count and classify the blood cells on a slide it is necessary to successively find each blood cell and focus its image on the vidicon target. An automatic focusing system is described in copending application Ser. No. 543,515, filed Jan. 23, 1975, Amos et al. U.S. Pat. No. 3,864,564 issued to W. J. Adkins discloses an acquisition system for successively locating a white blood cell and bringing it into proper position for analysis. That system includes a rotating multifaceted mirror which, in conjunction with a microscope, causes a first light sensor to scan the microscope slide and generate an acquisition signal. A second light sensor receives light pulses from the rotating mirror and generates sync pulses for a logic circuit to which the first sensor is connected. In response to the acquisition and sync pulses, the logic circuit produces signals which are used to drive a microscope stage in x and y directions to cause the white blood cell to be positioned within a small aperture.
Because of the stain used on the blood smear and because of the optical filter disposed in the optical system prior to the acquisition aperture, the white blood cells absorb more light than the red blood cells. Thus, the acquisition signal comprises a series of negatively going pulses superimposed on a positive bias voltage, provided that the first sensor generates a voltage that increases as the intensity of the applied light increases. The pulses generated in response to the white blood cells are ideally more negative than those generated in response to the red blood cells by an amount sufficient to enable a pulse discriminator circuit to readily distinguish between the two and provide an output pulse only in response to the white blood cell. Under such ideal conditions a comparator circuit, which compares the acquisition signal with a predetermined dc level, is adequate.
However, the light is not necessarily uniform over the field, and a red blood cell located at the end of the field can appear almost as dark as a white blood cell located at the center of the field. Also, the total light level seen by the first light sensor can vary such that the overall amplitude of the signal generated thereby also varies. Moreover, the light level across the field can vary considerably if the condenser optics are slightly misaligned. Thus it is possible for a red blood cell in one part of the field to appear as dark as a white blood cell in another part of the field and for the level of either to vary from time to time. Under such adverse conditions a circuit which employs a fixed comparison voltage may not be able to distinguish between pulses produced in response to white and red blood cells.